In response to the demands of consumers who are driven both by ever-escalating fuel prices and the dire consequences of global warming, the automobile industry is slowly starting to embrace the need for ultra-low emission, high efficiency cars. While some within the industry are attempting to achieve these goals by engineering more efficient internal combustion engines, others are incorporating hybrid or all-electric drive trains into their vehicle line-ups. To meet consumer expectations, however, the automobile industry must not only achieve a greener drive train, but must do so while maintaining reasonable levels of performance, range, reliability, and cost.
In recent years, electric vehicles have proven to be not only environmentally friendly, but also capable of meeting, if not exceeding, consumer desires and expectations regarding performance. While early electric vehicles used DC motors in order to achieve the variable levels of speed and torque required to drive a vehicle, the advent of modern motor control systems utilizing direct torque control have allowed AC motors to deliver the same level of performance while providing the many benefits associated with AC motors including small size, low cost, high reliability and low maintenance.
A variety of techniques are currently used to manufacture the rotor assembly in an AC motor, these techniques offering a range of performance capabilities. Regardless of the manufacturing technique, in general the rotor assembly consists of a plurality of laminated discs that are combined to form a stack. The laminated discs within the stack include a plurality of peripherally spaced openings or slots. Passing through each set of openings or slots is a metal conductive bar, typically fabricated from either aluminum or copper. The openings or slots may be aligned so that the conductive bars are parallel to the axis of the rotor assembly, or they may be slightly skewed causing the conductive bars to be oblique to the axis of the rotor assembly. The conductive bars may either be cast in place or pre-fabricated and inserted into and through the stack of laminated discs. At either end of the rotor assembly is an end ring formed by mechanically and electrically joining together the ends of the conductive bars that extend beyond the stack. In a conventional rotor assembly, the conductive bars and the end rings are typically either brazed or electron-beam welded together.
An example of a conventional rotor assembly is provided in U.S. Pat. No. 4,064,410. As disclosed, the rotor assembly is formed by inserting a plurality of arcuately spaced apart conductive bars through a stack of laminated discs. The end rings, which are disposed at opposite ends of the stack, are welded to the protruding end portions of the bars.
While conventional rotor assemblies typically use end rings that are fabricated separately from the conductive bars, die casting techniques may be used to cast the conductive bars and the end rings in a single operation. For example, U.S. Pat. Nos. 2,607,969 and 2,991,518 disclose conventional and vacuum-assisted die casting techniques, respectively, used to cast rotor assemblies from a variety of conducting metals. However due to the higher melting temperature and the greater density of copper, the techniques disclosed in these patents are best applied to aluminum castings. U.S. Pat. No. 5,332,026 discloses an improvement in the casting system that is designed to compensate for some of the unusual problems associated with die casting copper, thereby allowing rotor cage electrical conductivity of 95% or greater to be achieved in a die cast rotor.
In order to decrease the electrical resistance associated with the end ring assemblies, U.S. Pat. No. 8,365,392 discloses a rotor assembly in which slugs are brazed between the end portions of the rotor bars, the braze joints contacting a large percentage of the rotor bar end portions. After each rotor bar/slug assembly is heated to form a plurality of braze joints, each of the two rotor bar/slug assemblies is machined to remove a circumferential edge portion. Then, in at least one embodiment, a containment ring is fit over the machined regions of each rotor bar/slug assembly.
While there are a variety of techniques that may be used to fabricate the rotor assembly of an electric vehicle's motor, there are trade-offs associated with each approach between manufacturing complexity and cost and the resultant rotor's electrical and mechanical characteristics. Accordingly, what is needed is a rotor manufacturing process that is both cost effective and capable of yielding a structurally robust rotor that exhibits excellent electrical characteristics. The present invention provides such a manufacturing process.